1. Field of the Invention
The present invention relates generally to the field of optical transceivers. More specifically, the present invention discloses a full-duplex optical transceiver using a fixed telescope and a single steering mirror in which separate regions of the steering mirror are used for transmitting and receiving.
2. Statement of the Problem
In recent years, technological advancements in the efficiency, reliability, and manufacturability of lasers and optical detectors have led to the viability of laser communication systems as an alternative to conventional radio frequency (RF) systems. One of the inherent advantages of optical communications over RF stems from the relatively shorter wavelengths of the electromagnetic energy involved. Because of diffraction effects, a shorter wavelength allows the transmitted energy to be focused into a much smaller beam. This delivers the transmitted energy more efficiently to the remote communication terminal.
Past development efforts in optical communications have attempted to fully capitalize on this advantage. Systems have been proposed that attempt to collimate the laser energy into a beam width on the order of 1 to 10 micro-radians. Using currently realizable lasers, such systems are designed to support communications over ranges greater than 10,000 miles and at data rates exceeding hundreds of megabits per second. However, several disadvantages result. First, relatively large transmitter optics are required to obtain such tightly focused beams due to diffraction. Second, the need to point the beam and stabilize it to a fraction of the beam width (i.e., less than 1 95 micro-radian) leads to complex beam control systems and large gimbaled mechanisms. Third, the ultimate ability to deliver the transmitted energy to the remote communication terminal requires precise knowledge of the position of the remote communication terminal, which is usually obtained from tracking the signal received from the remote communication terminal. The transmit and receive optics must also be separated to prevent the local transmitted energy from affecting the local receiver, particularly if full-duplex communication is required. It then becomes necessary to mechanically maintain a very tight boresight tolerance to some small fraction of the transmit beam between the transmitter optics and the receiver optics. All of these factors result in heavy, high-power, complex, and costly designs. Such systems can still offer advantages over RF systems in applications involving long range, high data rate communications. However, in applications requiring much lower transmission ranges (i.e., less than 10,000 miles) and lower data rates, optical communication systems of the type described above do not favorably compare to RF systems in terms of power, weight, or cost.
A variety of different types of optical communication system have been invented in the past, including the following:
______________________________________ Inventor U.S. Pat. No. Issue Date ______________________________________ Grossman 3,813,553 May 28, 1974 Waddoups 3,989,942 Nov. 2, 1976 Sepp et al. 4,731,879 Mar. 15, 1988 Solinsky 5,060,304 Oct. 22, 1991 Solinsky 5,142,400 Aug. 25, 1992 ______________________________________ Bondurant et al., "An Opto-Mechanical Subsystem for Space Based Coherent Optical Communication," pages 92-100, SPIE Vol. 996 High Data Rate Atmospheric and Space Communications (1988). Barry et al., "1000-Mbits/s Intersatellite Laser Communication System Technology," pages 470-478, IEEE Transactions on Communications (April 1976). ______________________________________
Grossman discloses a laser transceiver that uses a single objective lens and a mechanism similar to a single lens reflex camera for switching between transmitting and receiving modes of operation.
Waddoups discloses a retro-reflecting laser responder. The receiving apparatus includes two tracking mirrors 22, 23 and a telescope with Cassegranian optics. The received beam is split by a partially silvered mirror 34. A portion of the beam is modulated by an electro-optic modulator 29, and a portion is reflected to a tracking detector 37.
Sepp et al. disclose another example of a laser communications system (IFF) using a modulated retro-reflector.
The Solinsky patents disclose an optical transceiver system using a matched pair of reflecting telescopes, one each for transmitting and receiving. The telescopes use Cassegranian optics with an additional retro-reflector behind the aperture of the primary reflector. The received signal 34 is used both for demodulating data and tracking.
The paper by Bondurant et al. describes the opto-mechanical subsystem of the Laser Intersatellite Transmission Experiment (LITE) carried out by the MIT Lincoln Laboratory. The system uses telescope optics and a steerable coarse pointing mirror (CPM) with a common optical path for both transmitting and receiving data. FIG. 6 of this paper discuss the spatial acquisition sequence used to establish a communication link between two units by means of a broadened beam followed by a series of progressively narrower beams.
The paper by Barry et al. describes an intersatellite laser communication system developed by McDonnell Douglas Astronautics Company for the U.S. Air Force. A schematic representation of the optical configuration of the system is shown in FIG. 7 of the paper, including a telescope assembly and a plurality of mirrors for tracking and alignment.
3. Solution to the Problem
None of the prior art references uncovered in the search show an optical transceiver using a single steering mirror in which separate regions of the mirror are used for transmitting and receiving. In addition, the use of a divergent beam greatly relaxes the tracking and pointing tolerances required for the transceiver and permits the design to be much simpler, more compact, lighter weight, and less costly than previous designs.